Display units for displaying outputs from data processing devices have traditionally been analog in nature. The most typical example is a Cathode Ray Tube (CRT) display used in the common personal computer (PC). In such analog displays, analog interfaces in the PC's video driving unit provide three analog signals (red, green and blue) to the analog display. Each analog signal can assume, in this example, 1 out of 64 possible analog levels. Thus, the analog display is capable of displaying 262,144 different colors (64.times.64.times.64).
As shown in FIG. 1, the conventional analog interface includes a video memory 1 storing a digital label for uniquely indicating each of 256 possible colors which can simultaneously be used for display. The digital label contains 8-bits so that it can uniquely designate 256 different colors. The 8-bit digital label is output from video memory 1 onto line 4 to color palette chip 2. For each of the 256 digital labels, color palette chip 2 stores a set of three digital values, one for each of the primary display colors red (R), green (G) and blue (B) that make up the corresponding color. Each of the three digital values is a 6-bit digital word thus allowing for 64 different levels. The three 6-bit words are output from the color palette chip 2 via a built-in digital-to-analog (D/A) converter and sent as three 64-level analog signals 5, 6 and 7 to analog display 3 (such as a CRT).
In operation, the PC is running an application program which requires that a certain color be displayed. The application program tells the PC's video display device driver (in the ROM BIOS) to display that certain color. Video memory 1 then outputs the 8-bit digital word on line 4 indicating that color. Color palette chip 2 receives the 8-bit word and outputs three analog 64-level signals 5 (for R), 6 (for G) and 7 (for B) to the analog display 3.
There are only 256 colors stored in the palette chip, yet there is a total of 262,144 colors available. The user can custom-design the palette chip 2 to store different colors therein. Alternatively, the application program itself can write directly to the palette chip 2 to change its colors if the application program requires a certain set of colors to be used.
Digital displays have recently been growing in popularity, especially for portable uses such as in notebook computers. The most typical example of a digital display is an LCD panel. Digital displays typically require that the R, G and B driving signals represent a smaller number of total levels than in an analog display. For example, in a digital display each of the R, G and B driving signals represent one of only 16 different levels (as opposed to one of 64 in the analog display). A 4-bit digital word (allowing 16 levels) is thus used in a digital display, while a 6-bit digital word (allowing for 64 levels) is used in the digital portion of the interface circuitry for driving the analog display. The digital display of our example is thus capable of displaying only 4,096 colors (16.times.16.times.16) as opposed to the 262,144 colors (64.times.64.times.64) displayable by the analog display.
Despite these differences, it is highly advantageous to drive a digital display using existing standard analog driver interfaces, such as the one shown in FIG. 1. This eliminates the need to provide a dedicated digital driving interface, and efficient use can be made of pre-existing circuitry without hardware modification.
However, to do so requires that the analog interface's 6-bit primary color signals be reduced to 4-bit signals so as to be compatible with the digital display.
One common way of making this reduction is simply to ignore the 2 least significant bits during the analog-to-digital (A/D) conversion that occurs inside the digital display. This, however, can produce undesirable effects since the analog signal will contain some noise and the effect of this noise will be greatly amplified by ignoring these 2 bits. Although the noise is only equivalent to a transition in the least significant bit it may actually have an effect on the fourth highest bit. That is, if the analog interface drives the 6-bit digital word 010011, the display should see this as 0100 (i.e. the least significant 2 bits are ignored). If noise equivalent to 000001 is present it will increase the value of the 6-bit digital word to 010100 and the digital display will see 0101 (instead of the desired 0100). Thus, a small amount of noise is greatly amplified and results in a different color being displayed and the error is clearly visible to the user.
There is thus a need for an improvement to the analog interface driving a digital display so that less noise (in the guise of the wrong color being displayed) is perceptible to the user.